Modern piezoelectric materials open possibilities to the evolution of acoustic transduction systems. In particular, the manufacture of thin piezoelectric sheets of polyvinylidene fluoride (PVDF) has been determinant for the development of new kind of sound and vibration generating elements. Reasons for this are related to the properties of this material, including its flexibility, ruggedness, softness, light weight, as well as its strong piezoelectricity.
In the development of piezoelectric actuators and, in particular, sound emitting drivers, the applications of said materials are somewhat more limited. One of the reasons for this is that even though the progress in the development of the piezoelectric materials has been rather significant, the small active sound generating movements, i.e. low displacements provided by the diaphragms made of such materials,.do not enable the generation of high sound levels. Especially in the low frequency range, the sound levels created with prior art type piezoelectric drivers cannot compete with those generated with other well-known, typically electrodynamic transduction principles. Still, piezoelectric sound transducers find many applications in areas, where especially small size, low power consumption and economical construction are more important than high volume level sound reproduction.
Sound transducers employing radiating surfaces arranged in a cylindrical or tubular form are already known in the art for certain applications.
U.S. Pat. No. 5,132,942 discloses an electroacoustic transducer, which utilizes a sound generating vibratory unit formed of a circular piezoelectric stack consisting of several hollow ceramic cylinders fitted inside an outer metal sleeve tube. The interior space within the piezoelectric stack is a closed space filled with air or air entraining foam in order to provide a medium that reflects interiorly directed sound waves outwards. The transducer operates omnidirectionally emitting the sound waves in the direction of the radius of its cylindrical shape. Because of its capability to provide high energy, low frequency sound waves, said transducer is especially suitable for geological and other exploration type applications, where hermetically potted, immersed transducers are required.
U.S. Pat. No. 3,978,353 presents a speaker system with a piezoelectric diaphragm supported in a cylindrical, radially sound emitting form and provided with a plurality of vibration regions arranged along the circumference of the cylindrically shaped structure in order to control the directional patterns of the speaker. Despite of the improved control over the directional properties, by its basic character the speaker system still remains as a radially emitting structure, which for many applications suffers from the inherent limitations of such structure.
The sound transducers described in the following patents employ cylindrical or tubular diaphragms, but instead of being radially sound emitting, they are arranged to radiate sound axially.
U.S. Pat. No. 6,532,292 describes a method and apparatus to transmit audio signals into a human ear. The apparatus comprises a first frequency generator driving a first cylindrical element, said first element generating a first ultrasonic acoustic field. The apparatus further comprises a second frequency generator driving a second cylindrical element for generating a second ultrasonic acoustic field. Said second element is positioned concentrically respect to said first element. The second ultrasonic acoustic field and first ultrasonic acoustic field are both directed into the ear canal of the listener, where said fields interact with each other producing audible modulation that can be detected by the ear. Instead of being radially sound emitting, the concentric elements in this solution may be characterized to be axially radiating. The main object of U.S. Pat. No. 6,532,292 is to provide an apparatus, which generates audio signal inside a listener's ear canal, and thereby effectively reduces echo and better ensures the privacy of the audio signal. The major drawback of the solution according to U.S. Pat. No. 6,532,292 is related to the fact that, correspondingly, it always requires the presence of the ear canal. In other words its applications are merely limited to different type of ear pieces. The scheme also requires a rather complicated driving scheme including frequency filtering and mixing circuits in other to accomplish correct type of modulation between the acoustic fields. Taken into account this and also the variation of the acoustic properties of individual human ear canals, a high quality and high level sound reproduction is likely to be rather challenging.
U.S. Pat. No. 3,859,477 describes another sound transducer having a cylindrical overall shape and emitting sound from the frontal ends thereof. Here, first a sheet-like assembly comprising at least a fixed electrode, a sound generating diaphragm and an intermediate spacer is constructed. Then, the transducer is constructed from the aforementioned assembly by spirally winding said assembly around a central core and into a cylindrical shape. Consequently, a rather large effective area of a diaphragm can be realized in a rather compactly sized transducer. However, the manufacturing of such transducers requires the winding of the sheet-like assembly in a specified spiral-like manner in order to produce transducers with consistent acoustical properties. The spirally wound construction also involves some significant limitations with respect to the acoustical properties of the transducer, especially with respect to the acoustic mass experienced by the diaphragm. The concept of acoustic mass will be discussed in more detail later in this text.